The goal of the research proposed is to understand, at the level of individual cells and fibers, the substrate of the functional reorganization that follows neonatal deafferentation of the hamster's superior colliculus (SC). The particular experiments outlined in this application are designed to examine three aspects of the collicular reorganization that occurs after neonatal enucleation of one eye. These are: 1. Reorganization of the axon arbors of individual, functionally characterized retinal and subcortical somatosensory afferents, 2. Effects of deafferentation upon structure-function relationships for postsynaptic cells, and 3. The role which sprouting of noradrenergic (NA) afferents plays in deafferentation induced functional reorganization. Extracellular recording studies have already shown that neonatal enucleation markedly alters the response properties of many SC neurons and "bulk tracing" experiments have demonstrated further that part of this reorganization might be explained by alterations in the terminal fields of several of the residual projections to the tectum. We will use intra-axonal recording and horseradish peroxidase (HRP) injection techniques to determine whether the expansion of the remaining retinal and subcortical somatosensory inputs to the SC reflects collateral sprouting; i.e., increases in the arbors of individual axons, "spreading" of azon terminals, i.e., an increase in the extent of the terminal arbors of a given axon will a concomitant decrease in their density or reduced overlap of normally sized arbors from different axons. We will also employ intracellular recording and HRP injection to define the receptive fields and structure of postsynaptic cells in the deafferented SC. A number of morphological studies have suggested that changes in the dendritic architecture of partially deafferented neurons may also play an important role in functional reorganization. Our experiments will test this hypothesis directly at the level of individual cells. In a third set of experiments, we will examine the role that sprouting of NA axons plays in the physiological changes observed in the deafferented SC. Our previous studies have demonstrated that NA fibers sprout vigorously after neonatal enucleation and our working hypothesis is that these afferents may act to inhibit the expression of weak sensory inputs onto SC neurons.